Monolithic piezoelectric element

ABSTRACT

A monolithic piezoelectric element includes a stack, and the stack includes a crack-forming conductive layer arranged to intentionally form a small crack in the stack. The small crack alleviates stress, thereby preventing the occurrence of a large crack that may extend to the piezoelectrically active region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to monolithic piezoelectric elements, suchas monolithic piezoelectric actuators.

2. Description of Related Art

Monolithic piezoelectric elements are known as a type of piezoelectricelement that converts electric energy into mechanical energy using anelectrostrictive effect of solid material. The monolithic piezoelectricelement is formed by alternately stacking piezoelectric ceramic layersand internal conductive layers.

The monolithic piezoelectric element has a similar structure asmonolithic ceramic capacitors, and the internal conductive layers arealternately connected to one and the other external electrode. When avoltage is applied to the two external electrodes, an electric field isgenerated between each of the two adjacent internal conductors, andthus, distortion occurs in the ceramic layers.

However, the electric field is generated to cause distortion only in aregion (piezoelectrically active region) where the internal conductivelayers overlap when viewed in the stacking direction, and the distortiondoes not occur in the other region (piezoelectrically inactive region),where the internal conductors do not overlap and hence no electric fieldis generated.

Consequently, when a large distortion is produced by applying a voltage,a high stress is generated at the boundary between the piezoelectricallyactive region and the piezoelectrically inactive region. The stress maymechanically break the stack. For example, a large crack may occurbetween the internal conductive layer and the ceramic layer.

In order to prevent the stack from being broken by the internal stress,Japanese Examined Patent Application Publication No. 6-5794 hasdisclosed that grooves extending in a direction parallel to the internalconductive layers are formed in a side surface parallel to the stackingdirection of the stack. Hence, the invention described in JapaneseExamined Patent Application Publication No. 6-5794 alleviates theconcentration of the stress by removing part of the piezoelectricallyinactive region.

Although Japanese Examined Patent Application Publication No. 6-5794does not disclose the process for forming the grooves in detail, theymay be formed by, for example, using ceramic green sheets on which avanishing material such as carbon paste has been printed. The vanishingmaterial is eliminated during firing, thus forming the grooves. Thegrooves may be cut in the stack with a wire saw after firing.

However, these methods have disadvantages.

In the method of eliminating the vanishing material to form the grooves,the piezoelectric ceramic layers are fused and bonded together afterdisappearance of the vanishing material because the vanishing materialdisappears at a temperature lower than the firing temperature of thepiezoelectric ceramic layers. Consequently, the grooves cannot be formedin a desired shape.

The method using a wire saw is inferior in processing precision, and itcannot be applied to the process using thin ceramic layers. In addition,the processing cost is high, and accordingly the manufacturing cost isincreased.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a monolithic piezoelectric element thatcan be easily manufactured while the stack is prevented from beingmechanically broken by stress.

A monolithic piezoelectric element according to a preferred embodimentof the present invention includes a stack including piezoelectricceramic layers and internal conductive layers that are integrallyformed, and external electrodes formed on the surface of the stack. Thestack further includes a crack-forming conductive layer arranged insidethe stack so as to form a crack in the stack.

By forming the crack-forming conductive layer, a tiny crack can beintentionally formed in the vicinity of the crack-forming conductivelayer, and thus stress can be reduced and dissipated. Thus, theoccurrence of a large crack that degrades the performance of the elementcan be prevented.

In the monolithic piezoelectric element according to a preferredembodiment of the present invention, the interfacial strength betweenthe crack-forming conductive layer and the ceramic layer may be lowerthan the interfacial strength between the internal conductive layer andthe ceramic layer.

Thus, a tiny crack can be preferentially formed at the interface betweenthe crack-forming conductive layer and the ceramic layer having a lowerinterfacial strength. The tiny crack can effectively prevent a largecrack that degrades the performance of the element.

The technique for reducing the interfacial strength between thecrack-forming conductive layer and the ceramic layer to lower than theinterfacial strength between the internal conductive layer and theceramic layer is not particularly limited. For example, it can berealized in a structure in which the internal conductive layers containa ceramic material having the same composition system as the ceramiccontained in the ceramic layers, and in which the crack-formingconductive layer does not contain the ceramic material having the samecomposition system as the ceramic material of the ceramic layer orcontains an amount of the ceramic material that is lower than the amountin the internal conductive layer.

By adding to a conductor a ceramic material having the same compositionsystem as the ceramic material contained in the ceramic layer, theinterfacial strength between the conductor and the ceramic layer isincreased. Accordingly, the interfacial strength between thecrack-forming conductive layer and the ceramic layer can be reduced tolower than the interfacial strength between the internal conductivelayer and the ceramic layer by adding the ceramic material to theinternal conductive layer to enhance the interfacial strength betweenthe internal conductive layer and the ceramic layer while thecrack-forming conductive layer contains the ceramic material in a lowerproportion or does not contain the ceramic material.

Ceramic materials having the same composition systems each other referto ceramic materials containing the same element as a principalconstituent. Preferably, the internal conductive layer contains aceramic material having the same composition as the ceramic materialcontained in the ceramic layers.

The crack-forming conductive layer may have a larger thickness than theinternal conductive layer.

As the thickness of a conductor is reduced, ceramic cross-links are moreeasily formed in the conductor, and accordingly, the interfacialstrength between the conductor and the ceramic layer is increased.Therefore, the interfacial strength between the crack-forming conductivelayer and the ceramic layer can be reduced to lower than the interfacialstrength between the internal conductive layer and the ceramic layer byincreasing the thickness of the crack-forming conductive layer so as tobe greater than the thickness of the internal conductive layer.

In the monolithic piezoelectric element according to various preferredembodiments of the present invention, the crack-forming conductive layeris preferably disposed along a surface of the layers of the stack, andmay be disposed on the same surfaces as the internal conductive layersor on surfaces on which the internal conductive layers are not formed.

Preferably, the crack-forming conductive layer is arranged to avoid theregion where the internal conductive layers overlap in a direction inwhich the layers are stacked, that is, the piezoelectrically activeregion. Thus, the internal stress produced at the boundary between thepiezoelectrically active region and the piezoelectrically inactiveregion can be eliminated, and cracks can be prevented from extending tothe piezoelectrically active region. Consequently, the degradation ofthe performance of the monolithic piezoelectric element can beprevented.

In the monolithic piezoelectric element according to various preferredembodiments of the present invention, when sections are defined bydividing the stack by the surfaces of layers on which the crack-formingconductive layer is disposed, the number of sections is M, and thedistortion in the stacking direction is D (μm), the distortion persection D/M may preferably be about 7.5 μm or less.

Thus, a large crack that leads to degradation of the performance of theelement can be effectively prevented from occurring.

According to preferred embodiments of the present invention, thepresence of the crack-forming conductive layer intentionally forms atiny crack to alleviate stress, and consequently prevents the occurrenceof large cracks and thus, the degradation of the element performance. Inaddition, since it is not necessary to use a vanishing material or toform a groove with a wire saw, the element can be easily manufactured bya known stacking process.

Other features, elements, steps, characteristics and advantages of thepresent invention will be described below with reference to preferredembodiments thereof and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a monolithic piezoelectric elementaccording to a first preferred embodiment of the present invention.

FIG. 2 is a fragmentary sectional view of the monolithic piezoelectricelement according to the first preferred embodiment of the presentinvention.

FIG. 3 is a perspective view of a process for manufacturing themonolithic piezoelectric element according to the first preferredembodiment of the present invention.

FIG. 4 is a perspective view of a modification of the monolithicpiezoelectric element according to a preferred embodiment of the presentinvention.

FIG. 5 is a sectional view of a monolithic piezoelectric elementaccording to a second preferred embodiment of the present invention.

FIG. 6 is a perspective view of a process for manufacturing themonolithic piezoelectric element according to the second preferredembodiment of the present invention.

FIG. 7 is a perspective view of a modification of the monolithicpiezoelectric element according to a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The best mode for carrying out the present invention will now bedescribed with reference to the drawings.

First Preferred Embodiment

FIG. 1 is a sectional view of a monolithic piezoelectric elementaccording to a first preferred embodiment of the invention. Themonolithic piezoelectric element preferably includes a stack 10 andexternal electrodes 21 and 22 disposed on surfaces of the stack 10. Thestack 10 includes integrally formed ceramic layers 11, internalconductive layers 12 and crack-forming conductive layers 13.

The ceramic layers 11 are preferably made of a piezoelectric ceramicmaterial, such as lead zirconate titanate (PZT), for example.

The internal conductive layers 12 preferably mainly contain a metal,such as Ag or Pd, and are alternately connected to one and the otherexternal electrodes 21 and 22, respectively.

The crack-forming conductive layers 13 preferably mainly contain ametal, such as Ag or Pd, and are disposed in regions (piezoelectricallyinactive regions) where the internal conductive layers 12 do not overlapin the stacking direction. The crack-forming conductive layers 13 aredisposed on the surfaces of the ceramic layers not having the internalconductive layers 12. Although FIG. 1 shows the internal conductivelayers 12 have substantially the same thickness as the crack-formingconductive layers 13, the crack-forming conductive layers 13 have alarger thickness than the internal conductive layers 12.

The external electrodes 21 and 22 preferably mainly contain a metal,such as Ag, and formed on surfaces of the stack 10.

FIG. 2 is a fragmentary enlarged sectional view schematically showingthe crack-forming conductive layer 13 and its vicinity. A tiny crack 14is formed at the interface between the crack-forming conductive layer 13and the ceramic layer 11 after polarization. This crack 14 does notextend to the piezoelectrically active region defined by overlapping theinternal conductive layers 12. There is no crack at the interfacebetween the internal conductive layer 12 and the ceramic layer 11. Thisis because the presence of the crack-forming conductive layer 13intentionally forms a tiny crack 14 at the interface between thecrack-forming conductive layer 13 and the ceramic layer 11, therebyreducing the stress.

A method for manufacturing the monolithic piezoelectric element will nowbe described with reference to FIG. 3.

First, predetermined weights of metal oxides, such as titanium oxide,zirconium oxide, and lead oxide, are weighed out, and the mixture iscalcined to yield a piezoelectric PZT ceramic material. Thepiezoelectric ceramic material is pulverized into piezoelectric ceramicpowder, and the ceramic powder is agitated and mixed with water or anorganic solvent, an organic binder, a dispersant, an antifoaming agent,and so on in a ball mill to yield a ceramic slurry.

The ceramic slurry is vacuum-defoamed, and is then formed intoapproximately 80 μm thick plain ceramic green sheets by the doctor blademethod.

Internal conductive layer patterns 41 are printed on some of the plainceramic green sheets with an electroconductive paste containing Ag andPd in a weight ratio of about 7:3 to prepare internal conductive layerceramic green sheets 31. In this instance, the printing conditions arecontrolled so that the paste of the internal conductive layer pattern 41is applied at a thickness of about 1.0 μm, for example.

Also, crack-forming conductive layer patterns 42 are printed on otherplain ceramic green sheets with an electroconductive paste containing Agand Pd in a weight ratio of about 7:3 to prepare crack-formingconductive layer ceramic green sheets 32. In this instance, the printingconditions are controlled so that the paste of the crack-formingconductive layer pattern 42 is applied at a thickness of about 2.0 μm,for example. The thicknesses of the internal conductive layer pattern 41and the crack-forming conductive layer pattern 42 refer to the metalthicknesses measured by fluorescent X-ray spectrometry.

The internal conductive layer patterns 41 and the crack-formingconductive layer patterns 42 are each in contact with at least one edgeof the corresponding ceramic green sheet so that they can be connectedto the subsequently formed external electrodes.

The internal conductive layer ceramic green sheets 31, the crack-formingconductive layer ceramic green sheets 32, and plain ceramic green sheets33 are stacked as shown in FIG. 3 to prepare a green stack. The internalconductive layer ceramic green sheets 31 are alternately stacked so thatthe internal conductive layer patterns 41 are led out alternately to theside ends in the lateral direction. Also, the internal conductive layerceramic green sheets 31 and the crack-forming conductive layer greensheets 32 are alternately stacked. The plain ceramic green sheets 33 aredisposed at both ends of the stack in the stacking direction.

The resulting green stack is heated to approximately 400° C. todebinder, and is then fired at approximately 1100° C. for about 5 hoursin a normal atmosphere to yield the stack 10. The resulting stack 10preferably measures approximately 7 mm by 7 mm and 30 mm in height, forexample.

Then, an electroconductive paste containing Ag is patterned to form theexternal electrodes 21 and 22 shown in FIG. 1. Subsequently, Ag nets arebonded to the external electrodes 21 and 22 with an electroconductiveadhesive to reinforce the external electrodes 21 and 22, and lead wiresare bonded to the external electrodes 21 and 22 with solder, but thenets and wires are not shown in the figure. The side surfaces of thestack 10 are coated with an insulating resin to insulate the internalconductive layers 12 exposed at the side surfaces of the stack 10.

The external electrodes 21 and 22 are connected to a direct currentsource and polarized at an electric field intensity of about 3 kV/mm ina thermostatic chamber of about 80° C., for example. Thus, themonolithic piezoelectric element shown in FIG. 1 is completed.

Samples were prepared in the following procedure for measuring theinterfacial strengths between the internal conductive layer and theceramic layer and between the crack-forming conductive layer and theceramic layer.

The same ceramic green sheets as used in the above-described monolithicpiezoelectric element were prepared, and internal conductive layerpatterns were printed at the same thickness in the same manner as above.Five ceramic green sheets with the internal conductive layer patternwere stacked, and 20 plain ceramic green sheets were pressure-bonded tothe bottom and top of the stack. The entire stack was fired under thesame condition as above. Then, the resulting sintered compact was cutinto samples of approximately 3 mm by 3 mm and 2.5 mm in height formeasuring the interfacial strength between the internal conductive layerand the ceramic layer.

Samples for measuring the interfacial strength between the crack-formingconductive layer and the ceramic layer were also prepared in the samemanner.

Each of the resulting samples was measured for the peel force betweenthe internal conductive layer and the ceramic layer or between thecrack-forming conductive layer and the ceramic layer when they wereseparated from each other using a tensile test apparatus, with a jigprovided to the top and bottom surface of the sample. Table 1 shows theresults, Weibull values m obtained by Weibull-plotting the measuredvalues and average strength μ.

TABLE 1 Weibull Average strength coefficient m μ (MPa) Internalconductive layer/ceramic layer 5.1 72.4 Crack-forming conductivelayer/ceramic 4.3 37.6 layer

It is found that the interfacial strength between the crack-formingconductive layer and the ceramic layer is reduced to less than theinterfacial strength between the internal conductive layer and theceramic layer by setting the thickness of the crack-forming conductivelayer 13 at twice the thickness of the internal conductive layer 12.

In addition, samples of a comparative example were prepared in the samemanner except that the crack-forming conductive layers are not provided(Comparative Example 1). The states of three monolithic piezoelectricelements of the present preferred embodiment and three monolithicpiezoelectric elements of Comparative Example 1 were observed through amicroscope after polarization.

In the monolithic piezoelectric element of the present preferredembodiment, 12.6 cracks were produced on average per element. Each crackoccurred in the vicinity of the crack-forming conductive layer and wasvery small. There was not a crack extending to the piezoelectricallyactive region.

On the other hand, 4.3 cracks were produced on average per element inthe monolithic piezoelectric elements of the comparative example, butabout one half of the cracks extended to the piezoelectrically activeregion.

Five monolithic piezoelectric elements each of the present preferredembodiment and the comparative example were subjected to a continuousdriving test at a temperature of about 30° C. and a humidity of about60% by applying rectangular waves of about 200 V in maximum voltage andabout 30 Hz in frequency.

All the five monolithic piezoelectric elements of the present preferredembodiment were normally operated after being driven 109 times. Thedistortions before and after the driving test were compared, and thedegradation in distortion was within 5% in all the samples. On the otherhand, all the monolithic piezoelectric elements of the comparativeexample were mechanically damaged, and were not operational after beingdriven 105 times.

Samples including the crack-forming conductive layers at intervalsvaried by thinning out some of the crack-forming conductive layers wereprepared. More specifically, each monolithic piezoelectric element wasprepared such that crack-forming conductive layers are not provided atthe interfaces between specific internal conductive layers and thus, theinterval between the crack-forming conductive layers was varied, whilein the structure shown in FIGS. 1 and 3, the ceramic layers 11 havingthe internal conductive layers 12 (internal conductor ceramic greensheets 31) and the ceramic layers 11 having the crack-forming conductivelayers 13 (crack-forming conductive layer ceramic green sheets 32) arealternately disposed. In this instance, plain ceramic layers weredisposed in regions from which the ceramic layers having thecrack-forming conductive layers were thinned out so that the intervalsbetween the internal conductive layers in the stacking direction werethe same. In this example, the ceramic layers 11 having thecrack-forming conductive layers 13 were arranged substantially equallyin the stacking direction of the stack.

Thus, seven types of samples were prepared. Five samples each of theseven types and Comparative Example 1 of the monolithic piezoelectricelement were subjected to the continuous driving test in the same manneras above.

The distortion per section, D/M, was calculated. The sections weredefined by dividing the stack by the surfaces of stacked layers on whichthe crack-forming conductive layers were disposed, and the number ofsections was designated by M.

When the number of surfaces on which the crack-forming conductive layersare disposed is N, M is expressed by equation (1). More specifically,when, for example, the crack-forming conductive layers are formed on twosurfaces of the stacked layers, M=3 holes. If there is no crack-formingconductive layer on the surfaces of stacked layers, M=1 holes.M=N+1  (1)

The results of measurement for distortion per section, D/M, andcontinuous driving test are shown in Table 2. Table 2 also shows thedimension of the section in the stacking direction, L/M (L represents adimension of the stack in the stacking direction) for reference. InSample 7, the crack-forming conductive layers are disposed in all theregions between two adjacent internal conductive layers.

TABLE 2 Number of Number of samples that crack-forming Dimension perDistortion per were able to be driven at Sample conductive section L/Msection D/M the point of driving times No. layers (mm) (μm) 10⁴ 10⁵ 10⁶10⁷ 10⁸ 10⁹ 1 2 10 15 5 5 5 3 1 0 2 5 5.0 7.5 5 5 5 5 5 4 3 7 3.75 5.6 55 5 5 5 5 4 9 3.0 4.5 5 5 5 5 5 5 5 11 2.5 3.8 5 5 5 5 5 5 6 14 2.0 3.05 5 5 5 5 5 7 273 0.1 0.16 5 5 5 5 5 5 Comparative 0 30 45 4 0 0 0 0 0Example 1

As shown in Table 1, one of the monolithic piezoelectric elements of thecomparative example did not work at e point of 104 times, and all ofthem did not work at the point of 105 times.

On the other hand, the monolithic piezoelectric elements of Sample Nos.1 to 7 according to preferred embodiments the present inventionexhibited enhanced durability against continuous driving in contrast toComparative Example 1. In Sample Nos. 3 to 7, particularly, all theelements were able to be driven even at the point of 109 times. Thissuggests that it is preferable that the crack-forming conductive layersare disposed so as to set the distortion between the crack-formingconductive layers to less than about 7.5 μm.

While the present preferred embodiment includes substantiallyrectangular crack-forming conductive layer patterns, the shape of thecrack-forming conductive layer patterns is not limited. For example, theshape may be semicircular, as shown in FIG. 4.

Second Preferred Embodiment

A monolithic piezoelectric element according to a second preferredembodiment of the present invention will now be described. Descriptionsof elements that are the same as or corresponding to those of the firstpreferred embodiment will be omitted as needed. FIG. 5 is a sectionalview of a monolithic piezoelectric element according to a secondpreferred embodiment.

The monolithic piezoelectric element includes a stack 10 and externalelectrodes 21 and 22. The stack 10 includes integrally formed ceramiclayers 11, internal conductive layers 12 and crack-forming conductivelayers 13. Although this structure is different from that of the firstpreferred embodiment in that the crack-forming conductive layer 13 andthe internal conductive layer 12 are disposed on the same surfaces ofthe stacked layers, and the other parts are the same as in the firstpreferred embodiment.

The process for manufacturing the monolithic piezoelectric element willbe described below with reference to FIG. 6. First, predeterminedweights of metal oxides, such as titanium oxide, zirconium oxide, andlead oxide, are weighed out, and the mixture is calcined to yield apiezoelectric PZT ceramic material. The piezoelectric PZT ceramicmaterial is pulverized into piezoelectric ceramic powder, and theceramic powder is agitated and mixed with water or an organic solvent,an organic binder, a dispersant, an antifoaming agent, and so on in aball mill to yield a ceramic slurry.

The ceramic slurry is vacuum-defoamed, and is then formed into plainceramic green sheets with a thickness of about 160 μm by the doctorblade method.

Now, an electroconductive paste for internal conductive layers isprepared by mixing Ag powder, Pd powder, the same piezoelectric ceramicpowder as used in the formation of the ceramic green sheets, and anorganic vehicle. Also an electroconductive paste for the crack-formingconductive layer is prepared by mixing Ag powder, Pd powder, and anorganic vehicle. The ratio of the Ag content to the Pd content isAg:Pd=7:3 in both pastes.

Internal conductive layer patterns 41 are screen-printed on some of theplain ceramic green sheets with the electroconductive paste for internalconductive layers. Crack-forming conductive layer patterns 42 arefurther screen-printed with the electroconductive paste for thecrack-forming conductive layers. Thus, conductor ceramic green sheets 34are prepared. The internal conductive layer pattern 41 and thecrack-forming conductive layer pattern 42 are led out to one and theother of the opposing side ends of the conductor ceramic green sheet 34,respectively.

The printing conditions are controlled so that the internal conductivelayer pattern has a thickness of about 1.4 μm and the crack-formingconductive layer pattern 42 has a thickness of about 2.0 μm, forexample. The thicknesses of the patterns refer to the metal thicknessesmeasured by fluorescent X-ray spectrometry.

The conductor ceramic green sheets 34 and plain ceramic green sheets 33are stacked as shown in FIG. 5 to prepare a green stack. The green stackis heated to approximately 400° C. to debinder, and is then fired atapproximately 1100° C. for about 5 hours in a normal atmosphere to yieldthe stack 10 shown in FIG. 4. The resulting stack 10 measuresapproximately 7 mm×7 mm and 30 mm in height, for example.

Then, an electroconductive paste containing Ag is patterned on surfacesof the stack 10 to form the external electrodes 21 and 22. Subsequently,Ag nets are bonded to the external electrodes 21 and 22 with anelectroconductive adhesive to reinforce the external electrodes 21 and22, and lead wires are bonded to the external electrodes 21 and 22 withsolder, but the nets and wires are not shown in the figure. The sidesurfaces of the stack 10 are coated with an insulating resin to insulatethe internal conductive layers exposed at the side surfaces of the stack10.

The external electrodes 21 and 22 are connected to a direct currentsource and polarized at an electric field intensity of about 3 kV/mm ina thermostatic chamber of about 80° C., for example. Thus, themonolithic piezoelectric element shown in FIG. 5 is completed.

Samples were prepared in the following procedure for measuring theinterfacial strengths between the internal conductive layer and theceramic layer and between the crack-forming conductive layer and theceramic layer.

The same ceramic green sheets as used in the monolithic piezoelectricelement were prepared, and internal conductive layer patterns wereprinted using the same internal conductive layer conductive paste asabove. Five ceramic green sheets with the internal conductive layerpattern were stacked, and 10 plain ceramic green sheets werepress-bonded to the bottom and top of the stack. The entire stack wasfired under the same conditions as above. Then, the resulting sinteredcompact was cut into samples of approximately 3 mm by 3 mm and 2.5 mm inheight for measuring the interfacial strength between the internalconductive layer and the ceramic layer.

Test pieces for measuring the interfacial strength between thecrack-forming conductive layer and the ceramic layer were also preparedin the same manner using the same crack-forming conductive layerelectroconductive paste as above.

Each of the resulting test pieces was measured for the peel forcebetween the internal conductive layer and the ceramic layer or betweenthe crack-forming conductive layer and the ceramic layer when they wereseparated from each other using a tensile test apparatus, with a jigprovided to the top and bottom surface of the test piece. Table 3 showsthe results, Weibull value m obtained by Weibull-plotting the measuredvalues and average strength μ.

TABLE 3 Weibull Average strength coefficient m μ (MPa) Internalconductive layer/ceramic layer 4.8 74.2 Crack-forming conductivelayer/ceramic 4.3 37.6 layer

It is found that the interfacial strength between the internalconductive layer and the ceramic layer is increased by adding to theinternal conductive layer a ceramic material having the same compositionas the ceramic material contained in the ceramic layer, in comparisonwith the interfacial strength between the crack-forming conductivelayer, which does not contain the ceramic material, and the ceramiclayer.

In addition, monolithic piezoelectric elements of a comparative examplewere prepared in which only the internal conductive layer patterns wereformed on the conductor ceramic green sheets, but the crack-formingconductive layer patterns are not formed (Comparative Example 2).

The monolithic piezoelectric elements of the present preferredembodiment and the monolithic piezoelectric elements of the comparativeexample were observed through a microscope after polarization. Fourcracks extending to the piezoelectrically active region were observed inthe monolithic piezoelectric elements of Comparative Example 2. As forthe monolithic piezoelectric elements of the present preferredembodiment, 14 cracks, more than in comparative example 2, wereobserved, but each crack was formed in the piezoelectrically inactiveregion around the crack-forming conductive layer. There was not a crackextending to the piezoelectrically active region.

TABLE 4 Number of samples that were able to be driven at the point ofdriving times 10⁴ 10⁵ 10⁶ 10⁷ 10⁸ 10⁹ Example 2 5 5 5 5 5 5 ComparativeExample 2 4 0 0 0 0 0

As shown in FIG. 4, one of the monolithic piezoelectric elements ofComparative Example 2 did not work properly at the point of 10⁴ timesdue to mechanical damage, and all the samples did not work properlyafter being driven 10⁵ times. On the other hand, all the samples of themonolithic piezoelectric element of the present preferred embodimentworked properly even after being driven 10⁹ times.

While the present preferred embodiment has substantially rectangularcrack-forming conductive layer patterns, the shape of the crack-formingconductive layer patterns is not limited. For example, it may besemicircular, as shown in FIG. 7.

The above-disclosed first and second preferred embodiments are simplyexamples of the present invention, and the invention is not limited tothe disclosed preferred embodiments. For example, the crack-formingconductive layer may be formed in any shape anywhere as long as it canform a tiny crack in the piezoelectrically inactive region withoutextending to the piezoelectrically active region. The technique forreducing the interfacial strength between the crack-forming conductivelayer and the ceramic layer to less than the interfacial strengthbetween the internal conductive layer and the ceramic layer is notlimited to the above method, and any technique may be applied. Inaddition, other modifications may be made without departing from thespirit and scope of the invention.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A monolithic piezoelectric element comprising: a stack includingpiezoelectric ceramic layers and internal conductive layers that areintegral with each other, and external electrodes disposed on a surfaceof the stack; and a crack-forming conductive layer disposed inside thestack and arranged to form a crack in the stack.
 2. The monolithicpiezoelectric element according to claim 1, wherein the crack formed bythe crack-forming layer is located only within a piezoelectricallyinactive area of the stack.
 3. The monolithic piezoelectric elementaccording to claim 1, wherein the crack formed by the crack-forminglayer is not located in a piezoelectrically active area of the stack. 4.The monolithic piezoelectric element according to claim 1, wherein aninterfacial strength between the crack-forming conductive layer and anadjacent one of the ceramic layers is lower than an interfacial strengthbetween one of the internal conductive layers and an adjacent one of theceramic layers.
 5. The monolithic piezoelectric element according toclaim 2, wherein the internal conductive layers contain a ceramicmaterial having the same composition system as the ceramic materialcontained in the ceramic layers, and the crack-forming conductive layerdoes not contain the ceramic material having the same composition as theceramic material of the ceramic layers.
 6. The monolithic piezoelectricelement according to claim 2, wherein the internal conductive layerscontain a ceramic material having the same composition system as theceramic material contained in the ceramic layers, and the crack-formingconductive layer contains the ceramic material in a lower content thanthe internal conductive layers.
 7. The monolithic piezoelectric elementaccording to claim 2, wherein the crack-forming conductive layer has alarger thickness than the internal conductive layer.
 8. The monolithicpiezoelectric element according to claim 1, wherein the crack-formingconductive layer is disposed along a surface of the layers of the stack,and lies on the same surfaces as the internal conductive layers.
 9. Themonolithic piezoelectric element according to claim 1, wherein thecrack-forming conductive layer is disposed along a surface of the layersof the stack, and lies on surfaces on which the internal conductivelayers are not formed.
 10. The monolithic piezoelectric elementaccording to claim 1, wherein the crack-forming conductive layer isdisposed in a region where the internal conductive layers do not overlapin a direction in which layers are stacked.
 11. The monolithicpiezoelectric element according to claim 1, wherein when sections aredefined by dividing the stack by the surfaces of layers on which thecrack-forming conductive layer is disposed, the number of sections is M,and the distortion in a direction in which layers are stacked is D (μm),the distortion per section D/M is about 7.5 μm or less.